for a few seconds, a small phone options menu will or at least , should appear. this menu allows you to power off your phone, turn off or on airplane mode or restart your phone. However, in my previous phone at some point this function simply stopped working and the phone wouldn't react to holding the power button. Unfortunately, restarting the phone or turning it off and on didn't help.
The only thing that worked was restoring factory settings. To do this, do go
Settings> Backup and reset> Reset Phone.
Before you do that, remember to backup your settings with either a third party app or use the default function also available under
Backup and reset> Backup and restore.